i like this idea and will participate at some point.
my personal adventure styles tend towards either
1 sandbox entirely (so needs to be somewhat open)
or
2 great grand quest (tm) so even bigger and more work than the above.

i like it overall, i have reservations regarding the fire demon.
as I understand it if 5 characters enter they will all lose 1 skill and 2 stamina for 5 rounds...
and THEN have to fight a skill 15 enemy?
after losing 5 Skill and 10 Stamina

my main issue with this is that the rest of the adventure is achievable by new characters but this one fight might not be (dependant on party composition, a wizard with lightning blast would potentially do the demon on his own?)

ambidextrous is really useful. I was very impressed when the wife and I playtested the well with two characters, one of whom had ambidex, as to exactly how well it worked out.
I realised after the session that our planning was not great, but the next game (the other dungeon in the AFF2 rulebook) was a lot more fluid.
essentially the priest with leather hauberk and large shield engaged as many enemies as possible, fought defensively, and the rogue-wizard went on a killing spree one-on-one on every enemy possible.
the priest engaged normally once no longer outnumbered.

How about replacing the demon elemental with a Devlin (page 36 OOTP) - Skill 10, stmaina 0, causes 2 stamina damage on a hit. Cannot be hurt by earthly weapons but being covered in water banishes it. Also maybe have an extra room with a well or an underground spring as a source of water.